1. Field of the Invention
The present invention relates to photoconducting detectors. Such detectors are widely used in optoelectronics for detecting electromagnetic radiation, more especially for detecting electromagnetic radiation whose wavelength falls within the visible light range and especially the infrared range.
It will be recalled first of all that photoconducting detectors use the increase in electric conductivity of a semiconductor when this latter is subjected to illumination by electromagnetic radiation such as defined above. When the detector is subjected to such illumination, an increase in the number of free carriers can be observed in the conducting band. Now, the number of free carriers is proportional to exp (-E.sub.G /kT) with:
E.sub.G the energy of the prohibited band PA1 k Boltzmann's constant and, PA1 T the temperature PA1 It can then be seen that the number of carriers also depends on the temperature. Consequently, a variation in the ambient temperature causes a variation in the number of carriers in the conducting band in the same way as illumination by electromagnetic radiation. The carrier variation due to the temperature becomes very rapidly equivalent to that due to illumination, particularly in the case of semiconductors with small prohibited band. In this case, in fact, the variation of the number of carriers in the conducting band with respect to the number of carriers already present is small. However, the variation due to the temperature is a variation which evolves slowly with time. Also, in order to be able to distinguish the variations in the number of carriers due to the temperature from those due to illumination, the illumination is modulated and so a useful modulated signal can be detected at the output.
However, this problem of variation in the number of carriers due to the temperature is very important when it is desired to detect a DC signal, namely when it is impossible to modulate the illumination.
2. Description of the Prior Art
With photoconducting detectors, the circuit shown in FIG. 1 is preferably used for reading the useful signal corresponding to the illumination received. This circuit comprises a voltage source V to which the photoconducting detector D formed by a semiconductor volume with two ohmic contacts and a load resistor Rc are connected in series. The useful signal is detected at the terminals of the load resistor, one of which corresponds to the node between the photoconductor and load, the other of which to a voltage source terminal.
With this type of circuit, the useful signal is superimposed on a voltage which is often much greater than said signal and, if it is desired to detect the DC signal, this voltage must be subtracted, which implies a great stability thereof.
Now, when the temperature changes, a variation in the number of carriers can be observed in the photoconducting detector as explained above, that is to say a variation of the resistivity which is inversely proportional to the number of free carriers and consequently a variation in the resistance of the detector which varies exponentially according to an approximate law of exp (-E.sub.G /kT) whereas the load resistor has negligible variation as a function of the temperature.
The purpose of the present invention then is to allow a DC signal to be read while overcoming this disadvantage.